554 5.5.1 Error: no valid recipients

Hi,
We're trying to do away with our old domain and start using a new one. I've forwarded all the email from the old domain to the new one, everything has been working fine but now i seem to be getting this error.

I cant seem to see any errors in mail.log but this only happens with this one domain. now emails are bouncing back

To: [email protected]
Subject: Mail delivery failed: returning message to sender
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:[email protected]
host mail.newdomain.com [xxx.xxx.xxx.xx]
SMTP error from remote mail server after RCPT
TO:<[email protected]>:
451 4.3.5 Server configuration error: retry timeout exceeded

Please change the hostname of the server and all occurrences of this domain in main.cf file to a subdomain. Use e.g. server1.yordomain.com instead of yourdomain.com. A domain that you want to use for email with virtual users may not be used as hostname of the server nor may it be listed in postfix main.cf. Then restart the server.